1. Field of the Invention
The present invention relates to a method for preparing nylon 6 fibers. In particular, the invention involves the preparation of nylon 6 copolymer containing 5-sulfoisophthalate salts.
2. Description of the Prior Art
Nylon fiber is widely used in the manufacture of carpets and other textile materials. In recent years, advancements in the properties and applications of PET are continuously in progress, which is threatening to the nylon market. However, nylon, which exhibits specific characteristics, is still applicable in several areas. Present techniques or further development for the preparation of nylon fibers have placed more emphasis on using specific comonomer and nylon monomers in synthetic reaction to prepare nylon comonomers. Resins or fibers made from the nylon comonomers exhibit good stain resistance to food coloring and soil. Related patens are U.S. Pat. Nos. 5,889,138, 5,108,684 and JP Laid-open Patent No. 03-137221, 01223908, 53-142499 and 53-134896.
Conventional dyes used for nylon 6 are mainly acid dyes. Nylon copolymer modified using the above method can be dyed with cationic dyes, which allows further development of various kinds of new textile, disclosed in U.S. Pat. Nos. 5,484,455, 5,342,417 and 5,199,958.
The primary comonomer of the nylon copolymer is 5-sulfoisophthalate salts (or methylester). The nylon copolymer is prepared by adding 5-sulfoisophthalate salts (or esters) and hexamethylene diamine to the nylon comonomer (caprolactam; CPL) or its oligomer to promote copolymerization.
However, the water extractable compounds of the nylon copolymer obtained using the above method is too high, which causes: (1) large variation in the amount of the comonomer (for example, 5-sulfoisophthalate salts) of the nylon copolymer, which affects the stability of the physical and chemical characteristics of the nylon; (2) large amounts of compounds present in the extract, which increases the difficulty of recycling nylon 6 monomer (CPL), thus creating high production costs.
In order to overcome the above problems, the invention provides a method for preparing 5-sulfobenzenediamide and a method using the 5-sulfobenzenediamide as an intermediate to prepare nylon 6 copolymer containing sulfonate salts. The intermediate of 5-sulfobenzenediamide with terminal amines is prepared by synthesizing 5-sulfoisophthalate salts and an aliphatic diamine of formula II, preferably 1,6-hexamethylene diamine. Next, caprolactam and an aliphatic diacid, preferably, adipic acid, are reacted to form an nylon 6 oligomer with a low molecular weight. The intermediate 5-Sulfobenzenediamide and catalyst are then added into the nylon 6 oligomer obtained in the previous step to cause a polymerization reaction to obtain nylon 6 copolymer containing 5-sulfoisophthalate salts.
In order to achieve the present invention, there is provided a method for preparing nylon 6 copolymer containing 5-sulfoisophthalate salts comonomers, comprising the steps of:
i. reacting (A) 5-sulfoisophthalate salts ester (formula I) 
with (B) aliphatic diamine (formula II)
NH2(CH2)nNH2xe2x80x83xe2x80x83(formula II)
in a molar ratio of 2xcx9c30, followed by completely removing the unreacted aliphatic diamine, to obtain (E) 5-sulfobenzenediamide compound (formula III); 
ii. reacting (C) caprolactam with (D) aliphatic diacid (formula IV)
HOOC(CH2)nCOOHxe2x80x83xe2x80x83(formula IV)
at 200xcx9c2600xc2x0 C. to form an oligomer with low molecular weight; and
iii. adding (E) 5-sulfobenzenediamide (formula III) and catalyst to the oligomer obtained in step (ii) to cause a polymerization reaction to obtain nylon 6 copolymer containing 5-sulfoisophthalate salts comonomer, wherein the molar ratio of E/C is 0.005xcx9c0.150 and the molar ratio of D/E is 1.05xcx9c4.00.
In the method provided in the present invention, R is xe2x80x94(CH2)nH and n is an integer of 1xcx9c6 in formula I. In formula II, III and IV, n is an integer of 1xcx9c12. In addition, M is Li, Na or K in formula I and III. Suitable catalysts are hypophosphorous acid, acetic acid, alkaline metal and alkaline-earth metal. Preferred amount of the catalyst is 0.9 mole % (relative to caprolactam).
According to the method for preparing nylon 6 copolymer containing 5-sulfoisophthalate salts, the water extractable compounds of the produced nylon 6 copolymer is greatly reduced, thereby improving the stability of the amount of monomer present in the copolymer. Overall stability of the physical and chemical characteristics of nylon is consequently improved. In addition, compounds present in the extract are greatly reduced, and the complexity of recycling nylon 6 monomer (caprolactam) is reduced, thus reducing the production costs.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention.